Method for selecting a frequency converter for a refrigerant compressor unit

ABSTRACT

In order to improve a method for selecting a frequency converter for a refrigerant compressor unit comprising a refrigerant compressor and an electric drive motor in such a way that the frequency converter is optimised for the application in question, it is proposed that a working state suitable for the operation of the refrigerant compressor unit is selected in an application field of an application diagram of the refrigerant compressor, that an operating frequency is selected for this selected working state, and that, on the basis of drive data, a working state operating current value corresponding to the selected working state and the selected operating frequency is ascertained for the operation of the refrigerant compressor unit

CROSS-REFERENCE TO RELATED PATENT APPLICATION

This application is a continuation of International application numberPCT/EP2017/077658 filed on Oct. 27, 2017.

This patent application claims the benefit of International applicationNo. PCT/EP2017/077658 of Oct. 27, 2017, the teachings and disclosure ofwhich are hereby incorporated in their entirety by reference thereto.

BACKGROUND OF THE INVENTION

The invention relates to a method for selecting a frequency converterfor a refrigerant compressor unit, comprising a refrigerant compressorand an electric drive motor.

Previously, the frequency converters for refrigerant compressor unitswere always selected such that the frequency converter did not limit thepotential working states of the refrigerant compressor.

The result of this is that, in the previously known methods forselecting the frequency converter, frequency converters that incurredunnecessary costs were always used.

SUMMARY OF THE INVENTION

The object of the invention is therefore to improve a method forselecting a frequency converter such that the frequency converter isselected so as to be optimised for the application in question.

In a method of the kind described in the introduction, this object isachieved in accordance with the invention in that a working statesuitable for the operation of the refrigerant compressor unit isselected in an application field of an application diagram of therefrigerant compressor, in that an operating frequency is selected forthis selected working state, and in that, on the basis of drive data, aworking state operating current value corresponding to the selectedworking state and the selected operating frequency is ascertained forthe operation of the refrigerant compressor unit.

The advantage of the solution according to the invention is consideredto lie in the fact that the working state operating current valuecreates a benchmark for the selection of the frequency converter, whichbenchmark makes it possible to determine the frequency converter in asimple way, for example insofar as the frequency converter to beselected must at least be able to generate a current at the outputcorresponding to the working state operating current value.

For example, with the solution according to the invention, the drivedata are determined beforehand, in particular also depending on therefrigerant, and in particular are stored for later use when selectingthe frequency converter.

The frequency converter may then be selected particularly easily if, onthe basis of the working state operating current value, the frequencyconverter of which the maximum converter current value is equal to orgreater than the working state operating current value is selected fromdata for those frequency converters that are available for selection.

For example, for this purpose the data of the potential frequencyconverters are compiled in a list, which in particular is available as astored file.

With this approach, the selection of the suitable frequency convertercan thus be optimised insofar as the suitable frequency converter isselected such that its maximum converter current value is sufficient toreliably operate the refrigerant compressor unit in the selected workingstate at the selected operating frequency, however an unnecessaryoverspecification of the frequency converter is avoided, and thereforethe frequency converter that is the most economical for reliableoperation is selected.

For the optimisation of the selection of the frequency converter, it isalso advantageous if the frequency converter of which the maximumconverter current value is closest to the working state operatingcurrent value is selected.

It is thus ensured that the frequency converter is not overspecified inrespect of the maximum converter current value.

With such a focusing of the configuration of the frequency converter inrespect of the maximum converter current value, it could be the casethat the frequency converter is unable to provide the start-up currentvalue for the refrigerant compressor unit.

For this reason, it is preferably provided that the frequency converteris selected such that its maximum converter start-up current value isequal to or greater than a start-up current value of the refrigerantcompressor unit.

In this case, a stored start-up current value is preferably consultedfor the selection of the frequency converter.

No further details have yet been provided in respect of the way in whichthe start-up current value is determined.

In accordance with an advantageous solution, the start-up current valueis determined experimentally and in particular is then stored so as tobe available for the selection of the frequency converter.

It is thus ensured that the start-up current value corresponds to theactual conditions of the refrigerant compressor unit.

In order to also furthermore optimise the selection of the frequencyconverter in respect of the start-up current value, it is preferablyprovided that the frequency converter is selected such that its maximumconverter start-up current value is as close as possible to the start-upcurrent value, such that the maximum converter current value of thefrequency converter is not unnecessarily overspecified in this respecteither.

It is particularly expedient if the frequency converter is selected suchthat its maximum converter current value is as close as possible to thehigher of the values of the working state operating current and start-upcurrent, such that the configuration of the frequency converter inrespect of its maximum converter current value is thus optimised to theselected working state.

No further details have yet been provided in respect of the drive data.

A solution that reflects reality particularly well provides that thedrive data are determined experimentally.

Furthermore, an advantageous solution provides that experimental drivedata for the possible operating frequencies to be selected are storedfor each working state of the refrigerant compressor unit.

Likewise, no further details have yet been provided in respect of theoperating frequencies that are to be selected.

In accordance with an advantageous solution, the operating frequency tobe selected lies in the range of from 0 hertz to 140 hertz.

The operating frequency to be selected preferably lies in the range froma cut-off frequency of the frequency converter up to a frequency of 140hertz, preferably up to 90 hertz.

The drive data also have not yet been specified in greater detail.

In accordance with an approach that is easily realised, the drive datacomprise the experimentally determined electrical power consumption foreach working state in the application field at the various operatingfrequencies.

In this case it is possible in particular to calculate the working stateoperating current value at the selected operating frequency on the basisof the experimentally ascertained electrical power consumption at theparticular operating frequency taking into consideration an equivalentcircuit of the drive motor of the refrigerant compressor unit.

The working state operating current value is calculated in particular bytaking into consideration the impedance of the equivalent circuit of thedrive motor in order to ascertain the working state operating currentvalue.

It is furthermore preferably provided that, in order to ascertain theworking state operating current value, the experimentally ascertainedpower consumption of the refrigerant compressor unit is compared withthe power consumption resulting from the equivalent circuit, and on thisbasis the slip or load angle is ascertained, such that all parametersfor complete calculation of the working state operating current valueare thus provided.

The working state operating current value can thus be ascertained inparticular on the basis of the ascertained slip or load angle and theimpedance of the equivalent circuit of the drive motor.

A further possibility for optimising the selection of the frequencyconverter is that of minimising the working state operating currentvalue by varying the output voltage of the frequency converter.

In other words, on the basis of the relationship with the working stateoperating current value, which relationship is defined by the equivalentcircuit, and on the basis of the output voltage of the frequencyconverter, it is possible to minimise the working state operatingcurrent value by changing the output voltage of the frequency converter,such that, when selecting or configuring the frequency converter, it ispossible to select a frequency converter or a setting of a frequencyconverter of which the output value has a value leading to a minimalworking state operating current value, and this may also be taken intoconsideration in turn when selecting the frequency converter.

For example, it is thus possible to select the most economical frequencyconverter possible.

No further specific details have yet been provided in respect of theexperimentally ascertained, stored drive data.

In accordance with an advantageous solution, the experimentallydetermined electrical power consumption of each working state in theapplication field at the particular operating frequency is captured, inparticular stored.

When ascertaining the working state operating current value, thecalculation thereof starting from the electrical power consumption isalso necessary, since merely the stored electrical power consumption forthe selected working state is available in a memory.

Alternatively, in accordance with another advantageous solution, theworking state operating current values calculated from theexperimentally ascertained power consumption are captured, in particularstored, for the particular working state and the particular operatingfrequency.

In other words, for each working state and each operating frequency, theworking state operating current values are already calculated andstored, such that, when selecting the frequency converter, the workingstate operating current values already stored can be directly accessedand there is no need to calculate them in addition prior to theselection.

In accordance with a further advantageous solution, the working stateoperating current value is experimentally ascertained and captured, inparticular stored, for each working state and for each operatingfrequency.

This approach is more complex in respect of the experimentalascertainment of the working state operating current value, however iteradicates the need to calculate the working state operating currentvalue from the electrical power consumption and to consult theequivalent circuit, and therefore may represent a favourable solution inspecific circumstances or with a specific kind of equivalent circuit.

Since the method according to the invention for selecting a frequencyconverter is limited to the working states of the refrigerant compressoravailable in the application field of the application diagram, it ispreferably provided that, on the basis of the maximum converter currentvalue of the selected frequency converter, the working states belongingto this maximum converter current value in the application field at aselected operating frequency are ascertained with the aid of the drivedata.

This ascertainment of the working states starting from the maximumconverter current value determined in accordance with the selection ofthe frequency converter has the great advantage that there may thus bedetermined the restrictions of the application field and of the workingstates achievable in the application field, these restrictions beingbrought about by the selection according to the invention of thefrequency converter.

It is preferably provided in this regard that the working statesascertained in respect of the maximum converter current value aredisplayed visually in the application diagram.

In particular, a conventional display unit is provided for this purpose,which on the one hand displays the application diagram and on the otherhand displays the working states forming a limitation of the applicationfield in the application diagram.

In conjunction with the previous explanation of the solution accordingto the invention, it is assumed that no further specifications apply forthe frequency converters available for selection.

This has the disadvantage, however, that, on account of the restrictionsof the application field, working states may occur in which the workingstate operating current value at high operating frequencies exceeds themaximum converter current value.

In the case of a conventional frequency converter, this usually leads toa transfer into a malfunction mode in order to protect the frequencyconverter.

In accordance with a particularly advantageous embodiment of the methodaccording to the invention, however, there are available for selectiononly frequency converters that comprise a frequency-limiting unit,which, at operating frequencies above a cut-off frequency, limits theoperating frequency in such a way that the maximum converter currentvalue of the frequency converter is not exceeded.

A frequency-limiting unit of this kind thus has the advantage that, inspite of the selection of the frequency inverter according to theinvention, working states of the refrigerant compressor unit whichcannot be achieved across the entire frequency range, in particular notat operating frequencies above the cut-off frequency, are still allowed,however, if such working states are in fact achieved, the frequencyconverter itself limits the operating frequency in such a way that thereis no transfer into the malfunction mode.

In particular, it is provided in this regard that the working stateoperating current value of the frequency converter is continuouslydetected by the frequency-limiting unit.

In this case, it is then possible in particular that the working stateoperating current value of the frequency converter is compared with acurrent reference value, and the operating frequency is limited to alimit frequency which is present when the current reference value isreached.

The current reference value in the simplest case is the maximumconverter current value.

In order, however, to also detect the situation in which a maximumcompressor operating current value defined specifically for therefrigerant compressor unit is not exceeded, it is preferably providedthat the frequency-limiting unit takes into consideration both themaximum converter current value and also the maximum compressoroperating current value as current reference value and determines thelimit frequency on the basis of the lowest of the maximum currentvalues.

It is thus ensured that the selected frequency converter does notmalfunction, even under the working states achievable only at certainoperating frequencies, but instead allows these working states of therefrigerant compressor to be achieved, although only in a limited rangeof the operating frequencies.

In addition, in the method according to the invention it is alsoprovided that there is available for selection only a frequencyconverter in which a voltage adjustment unit brings about an increase inthe output voltage with reference to the operating frequency,independently of a fluctuation of a mains voltage.

This solution has the advantage that the selected frequency converter,also in the event of a fluctuating mains voltage, in particular withfluctuations by up to 20%, does not change the increase in the outputvoltage of the frequency converter with reference to the operatingfrequency, which increase is essential for the flow in the drive motorof the refrigerant compressor unit, but instead keeps this increaseconstant.

This is achieved in particular in that an intermediate circuit voltageof the frequency converter is measured and, by way of a comparison withat least one reference value, a voltage curve of the output voltage ofthe frequency converter is corrected in order to keep the increase ofthe output voltage with reference to the operating frequency constant.

In this case, in particular the intermediate circuit voltage representsa voltage that is favourable for the method according to the invention,since it is proportional to the mains voltage and thus also directlyreflects the fluctuations of the mains voltage.

In addition, the invention relates to a method performed by a dataprocessing unit, which method comprises the method steps according toany one of claims 1 to 28.

In addition, the invention relates to a computer program productcomprising commands which, when the program is run by a computer, causethe computer to carry out the method according to any one of claims 1 to28.

The invention furthermore relates to a data processing unit according tothe features of claims 29 to 55, wherein, with regard to the advantagesof the data processing unit, reference is made to the correspondingdescriptions of the method according to the invention.

The invention also relates, independently of the above-describedsolutions or also in combination therewith, to a refrigerant compressorsystem comprising a refrigerant compressor unit having a refrigerantcompressor and an electric drive motor and also comprising a frequencyconverter for operating the electric drive motor, wherein the frequencyconverter comprises a frequency-limiting unit, which at operatingfrequencies above a cut-off frequency limits the operating frequency insuch a way that the maximum converter current value of the frequencyconverter is not exceeded.

A frequency-limiting unit of this kind thus has the advantage that, withit and without any particular intervention, operation of the refrigerantcompressor system in working states of the refrigerant compressor whichcannot be achieved across the entire frequency range, in particular notat all operating frequencies lying above the cut-off frequency, can beachieved with the available maximum converter operating current, sincewhen such working states are achieved the frequency converter itselflimits the operating frequency in such a way that there is no transferinto the malfunction mode.

In particular, it is provided in this regard that the working stateoperating current value of the frequency converter is detectedcontinuously by the frequency-limiting unit.

In this case, it is possible in particular that the working stateoperating current value of the frequency converter is compared with acurrent reference value, and the operating frequency is limited to alimit frequency which is present when the current reference value isreached.

The current reference value in the simplest case is the maximumconverter current value.

In order, however, to also detect the situation in which a maximumcompressor operating current value defined specifically for therefrigerant compressor unit is not exceeded, it is preferably providedthat the frequency-limiting unit considers both the maximum convertercurrent value and also the maximum compressor operating current value ascurrent reference value and determines the limit frequency on the basisof the lowest of the maximum current values.

It is thus ensured that the selected frequency converter does notmalfunction, even under the working states achievable only at certainoperating frequencies, but instead allows these working states of therefrigerant compressor to be achieved, although only in a limited rangeof the operating frequencies.

The invention also relates, independently of the above-describedsolutions or also in combination therewith, to a refrigerant compressorsystem comprising a refrigerant compressor unit having a refrigerantcompressor and an electric drive motor and also comprising a frequencyconverter for operating the electric drive motor, wherein the frequencyconverter comprises a voltage adjustment unit which controls an increaseof the output voltage with reference to the operating frequency suchthat this increase occurs independently of a fluctuation of a mainsvoltage.

This solution has the advantage that the selected frequency converter,even in the event of a fluctuating mains voltage, in particular withfluctuations by up to 20%, does not vary the increase in the outputvoltage of the frequency converter with reference to the operatingfrequency, which increase is essential for the flow in the drive motorof the refrigerant compressor unit, but instead keeps this increaseconstant.

This is achieved in particular in that the voltage adjustment unitdetects an intermediate circuit voltage of the frequency converter and,by way of a comparison with at least one reference value, the increaseof the output voltage of the frequency converter in the event ofdeviations from the at least one reference value is corrected in orderto keep constant the increase of the output voltage with reference tothe operating frequency.

In this case, in particular the intermediate circuit voltage representsa voltage that is favourable for the method according to the invention,since it is proportional to the mains voltage and thus also directlyreflects the fluctuations of the mains voltage.

The correction of the increase in the output voltage with reference tothe operating frequency can be achieved easily if the voltage adjustmentunit generates a proportionality factor with which the increase in theoutput voltage of the frequency converter is corrected.

No specific details have yet been provided in respect of the referencevalues.

It has proven to be favourable if the reference values used by thevoltage adjustment unit comprise at least one of the values as follows:a reference frequency, a proportionality factor, and an intermediatecircuit voltage setpoint value.

In accordance with a solution that is advantageous for the correction ofthe increase in the output voltage, the frequency converter comprises afrequency converter controller, which on the basis of a frequencyrequest signal generates a voltage control signal which is fed, inaddition to the frequency request signal, to an inverter stagecontroller of an inverter stage of the frequency converter, and thevoltage adjustment unit cooperates with the frequency convertercontroller in order to control the increase in the output voltage withreference to the operating frequency.

With regard to the configuration of the frequency converter controllerit is preferably provided that the frequency converter controller has aproportional member which, on the basis of the frequency request signalof the voltage control signal, generates the voltage control signal, andthat the voltage adjustment unit corrects a proportionality behaviour ofthe proportional member.

In particular, it is provided here that the proportionality behaviour ofthe proportional member is corrected using the proportionalitycorrection factor.

Further features and advantages are the subject of the followingdescription and the schematic illustration of a number of exemplaryembodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic depiction of a refrigerant circuit with arefrigerant compressor unit, operated by means of a converter;

FIG. 2 shows a schematic depiction of an application diagram of therefrigerant compressor unit with an application field which is enclosedby an application limit and which defines the allowed working states ofthe refrigerant compressor unit;

FIG. 3 shows a depiction of a curve of an output voltage of thefrequency converter with reference to an operating frequency and a curveof a working state operating current value with reference to theoperating frequency;

FIG. 4 shows a schematic depiction of a data processing unit for theoptimal selection of a frequency converter in accordance with a firstexemplary embodiment of the solution according to the invention;

FIG. 5 shows a depiction of an equivalent circuit of a drive motor,formed as an asynchronous motor, of the refrigerant compressor unit,showing the equations for a motor impedance, an electrical powerconsumption, and a working state operating current value at a certainoperating frequency;

FIG. 6 shows a schematic depiction of a method according to theinvention for ascertaining limitations of the application field causedby the selection according to the invention of the frequency converterin accordance with the first exemplary embodiment;

FIG. 7 shows a schematic depiction of a second exemplary embodiment of amethod according to the invention for selecting a frequency converter;

FIG. 8 shows a schematic depiction of the second exemplary embodiment ofthe frequency converter according to the invention when determining therestrictions of the application field;

FIG. 9 shows a schematic depiction of a data processing unit for theoptimal selection of a frequency converter in accordance with a fourthexemplary embodiment of the solution according to the invention;

FIG. 10 shows a depiction of an equivalent circuit of a drive motor,formed as a synchronous motor or permanent-magnet synchronous motor, ofthe refrigerant compressor unit, showing the equations for a synchronousgenerated voltage, an electrical power consumption, and a working stateoperating current value at a certain operating frequency;

FIG. 11 shows a schematic depiction of a method according to theinvention for ascertaining restrictions of the application field causedby the selection according to the invention of the frequency converterin accordance with the fourth exemplary embodiment;

FIG. 12 shows a schematic depiction of a fifth exemplary embodiment of amethod according to the invention for selecting a frequency converter;

FIG. 13 shows a schematic depiction of the fifth exemplary embodiment ofthe frequency converter according to the invention when determining therestrictions of the application field;

FIG. 14 shows a schematic depiction of a frequency converter with afrequency-limiting unit;

FIG. 15 shows a schematic depiction of a frequency converter with avoltage adjustment unit;

FIG. 16 shows a schematic depiction of a voltage control signal for thefrequency converter with reference to the operating frequency; and

FIG. 17 shows a depiction of output voltages of the frequency convertersimilarly to FIG. 3 in the case of a fluctuating mains voltage;

DETAILED DESCRIPTION OF THE INVENTION

A refrigerant circuit 10 depicted schematically in FIG. 1 comprises arefrigerant compressor unit 20, which has a refrigerant compressor 22and an electric drive motor 24 driving the refrigerant compressor 22,the refrigerant compressor 22 and the drive motor 24 for example beingsuitable for integration in a single unit.

The refrigerant compressor 22 in the refrigerant circuit 10 compressesthe refrigerant circulated in the circuit, the refrigerant then beingfed in the refrigerant circuit 10 to a heat exchanger unit 12 which isarranged on the pressurised side of the circuit and in which thecompressed refrigerant is cooled, in particular condensed, bydissipation of heat W.

The cooled, in particular condensed refrigerant is fed in therefrigerant circuit 10 to an expansion member 14, in which thecompressed, in particular condensed and pressurised refrigerant isexpanded and is then fed in the refrigerant circuit 10 to a heatexchanger unit 16, in which the expanded refrigerant is able to absorbheat W so as to thus provide its cooling effect.

The refrigerant expanded in the heat exchanger unit 16 is then fed againto the refrigerant compressor 22 and is compressed by the refrigerantcompressor 22.

The expanded refrigerant, which has already absorbed heat in the heatexchanger unit 16, is thus fed at an inlet 32 to the refrigerantcompressor 22 at a saturation temperature STE, is then compressed in therefrigerant compressor 22, and exits the refrigerant compressor at anoutlet 34 at a saturation temperature STA.

The refrigerant compressor 22, depending on its construction and therefrigerant, operates without sustaining damage only with certain valuepairs of the saturation temperature STE at the inlet 32 and thesaturation temperature STA at the outlet 34 of the refrigerantcompressor 22, these being defined by an application diagram 36 shown inFIG. 2, wherein in the application diagram 36 the saturation temperatureSTE at the inlet 32 is plotted on the X axis and the saturationtemperature STA at the outlet 34 is plotted on the Y axis.

In this case, in the predefined application diagram 36, which inparticular is also predefined depending on the refrigerant, all valuepairs of the saturation temperature STE at the inlet 32 and of thesaturation temperature STA at the outlet 34 of the refrigerantcompressor 22 that are permissible for the refrigerant compressor 22 liewithin an application field EF that is enclosed on all sides by anapplication limit EG.

Application Diagrams of this kind for refrigerant compressors areexplained by way of example in the book “Lexikon der Kaltetechnik”(Dictionary of Refrigeration Engineering) by Dieter Schmidt (publishedby C. F. Muller), to which reference is made in this regard.

The value pairs of the saturation temperature STE from the inlet 32 andthe saturation temperature STA at the outlet 34 that are permissiblewithin the application field EF each define a working state AZ of therefrigerant compressor 22 that can be achieved with the refrigerantcompressor 22 in question.

Since the refrigerant compressor 22 is driven by the electric drivemotor 24, each working state AZ requires a certain electrical powerconsumption PAZ of the drive motor 24.

The electrical power consumption value PAZ of the drive motor 24 isdependent here on the one hand on the particular working state AZ in theapplication field EF and on the other hand on the speed of therefrigerant compressor 22.

If the refrigerant compressor 22 is operated at different speeds bymeans of a frequency converter 40, the speed of the refrigerantcompressor 22 is thus proportional to the operating frequency f suppliedto the drive motor 24 by the frequency converter 40.

Thus, an electrical power consumption value PAZ is associated with eachworking state AZ within the application field EF at a certain operatingfrequency f.

However, the electrical power consumption value PAZ of the electricdrive motor 24 is dependent not only on the working state AZ of therefrigerant compressor 22, but also on the type of electric drive motor24 and the layout of the connection of the windings thereof to thefrequency converter 40.

In the depicted exemplary embodiment, it is assumed that the electricdrive motor 24 is an asynchronous motor, or perhaps a permanent-magnetmotor, the windings of which are connected to the frequency converter 40in a star layout.

This layout of connection of the drive motor 24 to the frequencyconverter 40 has the result that, as shown in FIG. 3, when the drivemotor 24 is operated with the frequency converter, the output voltageU_(FU) generated by the frequency converter 40, starting from theoperating frequency f=0, increases linearly as operating frequency fincreases until a cut-off frequency f_(ECK) is reached, above which theoutput voltage U_(FU) no longer increases, but has reached its maximumoutput voltage U_(FUMAX).

With a further increase in the operating frequency f to a maximumfrequency f_(max), the output voltage U_(FUMAX) at which the drive motor24 is operated remains constant.

The maximum operating frequency f_(max) of the frequency converter 40for operating the electric drive motor is on the one hand contingent onthe structure of the electric drive motor 24 and on the other hand onthe structure of the refrigerant compressor 22 and is usually aroundvalues of 80 hertz or less, whereas the cut-off frequency f_(ECK)usually lies in the range between 40 and 60 hertz.

In this operating mode of the electric drive motor 24, the operatingcurrent in the particular working state AZ is likewise dependent on theoperating frequency f, thus resulting in working state operating currentvalues I_(AZ) which are constant between the operating frequency f=0 tof_(eck), but increase further at operating frequencies f above thecut-off frequency f_(ECK), for example up to the maximum operatingfrequency f_(max).

The maximum output voltage U_(FUMAX), which is available at the outputof the frequency converter 40 for operating the drive motor 24, isproportional to the intermediate circuit voltage of the frequencyconverter 40 and thus proportional to the supply voltage of thefrequency converter 40.

As shown in FIGS. 2 and 3, the power consumption value P_(AZ1) in aworking state AZ1 of the application diagram 36 is for example greaterthan in a working state AZ2 of the application diagram 36, which, as inFIG. 3, has the result that the working point operating current valuesI_(AZ1) have higher values than the working point operating currentvalues I_(AZ2) in the working state AZ2.

It is thus shown in FIGS. 2 and 3 that the working state operatingcurrent values I_(AZ) made available by the frequency converter 40 aredependent on the working states AZ, and therefore the frequencyconverter 40, depending on the working state AZ, must be able togenerate working state operating current values I_(AZ) of differentsize.

The costs of the frequency converter 40 are dependent on the maximumconverter current value I_(FUMAX) that a frequency converter 40 can makeavailable, and the greater the maximum converter current value I_(FUMAX)the higher the costs.

If the selection of the frequency converter 40 in accordance with theworking state AZ_(s) intended by the user of the refrigerant compressorunit 20 when the frequency converter is used, which working state may befor example the working state AZ1 or AZ2, and the selected operatingfrequency f_(S) are now optimised, it is thus possible to optimiseselection of the frequency converter 40 by taking into account theworking state AZ_(S) intended by the user and the operating frequenciesf_(S) in that the frequency converter 40 is selected taking into accountthe intended working state AZ_(S) and operating frequency f_(S) in sucha way that the frequency converter 40 is selected so that the maximumconverter current value I_(FUMAX) is selected to be greater than theworking state operating current value I_(AZfs) necessary for theselected working state AZ_(S) at the intended operating frequency f_(S).

To do this, the working state operating current value I_(AZfs) must bedetermined.

The working state operating current value I_(AZfs) at the particularoperating frequency f_(S) is determined, as shown in FIG. 4, using adata processing unit 50, comprising an input unit 52, in particularcombined with a display unit 53, for displaying the application diagram36 and for selecting the working state AZ_(S) and the operatingfrequency f_(S).

For this purpose, the data processing unit 50 operates withexperimentally determined drive data to characterise the drive motor 24of the refrigerant compressor unit 20.

For example, in a first exemplary embodiment relating to an asynchronousmotor it is provided for the power consumption values P_(AZ) for theparticular working states AZ in the application field EF of therefrigerant compressor unit 20 at the particular operating frequency fto be determined experimentally and to be stored in a memory 54associated with the data processing unit 50 as experimental powerconsumption values P_(AZEXf) in the form of a power data field.

These electrical power consumption values P_(AZEXf) then provide thepossibility, taking into account the Steinmetz equivalent circuit forthe drive motor 24, shown in FIG. 5, and the known resistance values Rand reactance values X, which are stored in a memory 56 associated withthe data processing unit 50, of calculating the impedance Z according toformula (F1) of the drive motor 24 and then, comparing theexperimentally determined power consumption value P_(AZfs) at theselected operating frequency f_(S) with the theoretical powerconsumption value P_(AZ) of impedance Z, of ascertaining the slip siteratively from the formula (F2), and then using the slip s toascertain the working state operating current value I_(AZfs) from theformula (F3) at the particular selected operating frequency f_(S).

The relationships and formulas shown in FIG. 5 may vary slightly,depending on the approximations and assumptions made in the Steinmetzequivalent circuit.

A Steinmetz equivalent circuit with the associated formulas is describedin the book “THE PERFORMANCE AND DESIGN OF ALTERNATING CURRENTMACHINES”, by M. G. SAY, THIRD EDITION, 1958, in PITMAN PAPERBACKS,1968, SBN 273 401998, pages 270 ff.

A similar Steinmetz equivalent circuit with the corresponding formulascan be found on Wikipedia under “Induction motor”, as at 4 Apr. 2016,and the references cited there.

Proceeding from this working state operating current value I_(AZfs), theappropriate frequency converter 40 is now determined in that the maximumconverter current value I_(FUMAX) made available by the frequencyconverter 40 must be greater than the working state operating currentvalue I_(Az)fs ascertained for the particular working state AZ at theselected frequency f_(s).

As a further constraint for the frequency converter 40 that is to beselected, a start-up current value I_(ANLEX) for the particularrefrigerant compressor unit 20 is also used, which has likewise beenascertained experimentally and stored in a memory 58 and may, whereappropriate, be greater than the working state operating current valueI_(AZfs).

For starting up the refrigerant compressor unit 20, the frequencyconverter 40 is designed to be resistant to overload, such that amaximum converter start-up current value I_(FUANLMAX) that is greaterthan the maximum converter current value I_(FUMAX) is made availablebriefly, and for example may be 170% of the maximum converter currentvalue I_(FUMAX) for a period of three seconds.

In this way, for selection of the frequency converter 40, illustratedschematically in FIG. 4, it is relevant that the maximum convertercurrent value I_(FU) is greater than the working state operating currentvalue I_(Azfs), and the maximum converter start-up current valueI_(FUANLMAX) is greater than the start-up current value I_(ANLEX) of therefrigerant compressor unit 20, as illustrated for example in FIG. 3,however, the maximum converter current I_(FUMAX) and the maximumconverter start-up current I_(FUANLMAX) should be as close as possibleto the working state operating current value I_(AZfs) and the start-upcurrent value I_(ANLEX) in order to select a frequency converter withthe smallest possible maximum converter current value I_(FUMAXs), whichrepresents the most economical solution.

With a frequency converter 40 selected in this way, because of theselection method it is ensured that the frequency converter is able tooperate the refrigerant compressor unit 20 in the selected working stateAZ_(s), but a frequency converter 40 selected in this way does notensure that the refrigerant compressor 22 can consequently be operatedin all working states AZ within the application field EF.

Rather, this approach and the selection of the frequency converter 40such that the frequency converter need only be able to supply theworking state operating current I_(AZfs) and the start-up current valueI_(ANLEX) have the effect of restricting the application field EF.

In order to display to a user the restriction of the application fieldEF that results from the selection made of the frequency converter 40,as illustrated for example in FIG. 6, and on the basis of the maximumconverter current value I_(FUMAXs) of the selected frequency converter40 s, the working states AZ in the application field EF that areassociated with this maximum converter current value I_(FUMAX) areascertained for the selected operating frequency f_(s) or indeed forother operating frequencies f_(s)′, using the equivalent circuit of thedrive motor 24 illustrated in FIG. 5 with the known resistance values Rand the known reactance values X from the memory 56, and using theformulas for electrical power consumption and working state operatingcurrent I_(AZ) that are illustrated in FIG. 5 and are associated withthe equivalent circuit of the drive motor 24, taking into account thepower consumption values P_(AZEX) stored in the memory 54 for thedifferent working states AZ in the application field EF at theparticular selected operating frequencies f_(s).

For this purpose, the maximum converter current value I_(FUMAXs) of theselected converter 40 s is used for the current I_(AZfs) according tothe formula F3, the slip s is determined from this, and the formula F2is used to calculate the power consumption value P_(AZCAL), and then,using the experimental power consumption values P_(AZEX) stored in thememory 54, all the working states AZCAL(fs) that correspond to thecalculated power consumption value P_(AZCAL) at the selected operatingfrequency fs are ascertained.

The sum of these working states A_(ZCALfs) gives a boundary line G_(fs)in the application diagram 36, as illustrated in FIG. 2.

This calculation results in the boundary lines G_(fs) illustrated inFIG. 2 and FIG. 6 for different selected operating frequencies f_(s);for example the boundary line G_(fs) represents the boundary line forthe application field EF at the operating frequency f_(s) that isselected for selection of the frequency converter 40, the boundary lineG_(fr) represents for example a boundary line for the limit of theapplication field EF at a smaller operating frequency fr than theselected operating frequency fs, and the boundary line G_(frr)represents for example a boundary line of the application field EF foran operating frequency frr selected to be even smaller, and these aredisplayed by the data processing unit 50 on a display unit 53 togetherwith the application diagram 36.

Thus, a user of the method according to the invention is also at thesame time provided with information regarding the restrictions resultingfrom the selection of the frequency converter 40 in accordance with theselection method described above, and a user can check whether theserestrictions of the application field EF do or do not rule out possiblepotential working states AZ that could, where appropriate, also beapplicable for use of the refrigerant compressor unit 20.

In a second exemplary embodiment, as illustrated in FIG. 7, as analternative to the first exemplary embodiment, it is provided for thecurrent I_(Azf) to be ascertained in the manner described in conjunctionwith the first exemplary embodiment, using the data processing unit 50for each experimentally determined power consumption value P_(AZEXf) atthe particular operating frequency f, using the resistance values R andreactance values X of the Steinmetz equivalent circuit that are knownfrom FIG. 5 for each individual working state AZ, and to be stored in amemory 54′ such that when a user makes a selection of the working stateAZ_(s) and the selected operating frequency f_(s), the correspondingworking state operating current value I_(AZfs) may be accessed directlyin the memory 56, and this working state operating current valueI_(AZfs) corresponding to the selected working state AZ_(s) can be readoff directly without further action, and, using the experimentallydetermined start-up current value I_(ANLEX) the selection of thefrequency converter 40 s can be performed, using the maximum frequencyconverter currents I_(FUMAX) stored in the memory 62, in the manneralready described in conjunction with the first exemplary embodiment.

Similarly, in the second exemplary embodiment, once the frequencyconverter 40 s has been established, the maximum frequency convertercurrent I_(FUMAXs) may be used to determine the working statesA_(ZCALfs) associated with this current value in the memory 54′, and todisplay the sum of all these working states AZCAL_(fs) as the particularboundary line G_(fs) for example on a display unit 64, as described inconjunction with the first exemplary embodiment.

In a third exemplary embodiment, as an alternative to the first andsecond exemplary embodiments, it is provided, similarly to the secondexemplary embodiment, for the working state operating current valuesI_(Azf) to be determined experimentally in the memory 54′ and stored inthe memory 54′ such that in the third exemplary embodiment, in a similarmanner to the second exemplary embodiment, selection of the frequencyconverter 40 s can use the values in the memory 54′ as a starting point.

Similarly, and conversely, when determining the boundary lines G_(fs),the data processing unit 50 can proceed in accordance with the secondexemplary embodiment, with the experimentally determined working stateoperating current values I_(Azf) being stored in the memory 54′ and thenused to ascertain the boundary line G_(f) with the maximum convertercurrent value I_(FUMAXs) established by the selected frequency converter40 s.

The working state operating current value I_(AZfs) at the particularoperating frequency f_(s) is determined in a fourth exemplary embodimentrelating to a synchronous motor or a permanent-magnet synchronous motor,as shown in FIG. 9, using a data processing unit 50 comprising an inputunit 52, in particular combined with a display unit 53 for displayingthe application diagram 36 and for selecting the working state AZ_(s)and the operating frequency f_(s).

To do this, the data processing unit 50 uses experimentally determineddrive data in order to characterise the drive motor 24′ of therefrigerant compressor unit 20.

For example, in the fourth exemplary embodiment relating to asynchronous motor it is provided for the power consumption values P_(AZ)for the particular working states AZ in the application field EF of therefrigerant compressor unit 20 at the particular operating frequency fto be determined experimentally and to be stored in a memory 54associated with the data processing unit 50 as experimental powerconsumption values P_(AZEXf) in the form of a power data field.

These electrical power consumption values P_(AZEXf) then provide thepossibility, taking into account the equivalent circuit for the drivemotor 24′, shown in FIG. 10, and the known resistance values R andreactance values X, which are stored in a memory 56 associated with thedata processing unit 50, and comparing the experimentally determinedpower consumption value P_(AZfs) at the selected operating frequencyf_(S) with the theoretical power consumption value P_(AZ) in formula P2,of determining the load angle ϑ, iteratively using the formula P3, andof then using the load angle ϑ to ascertain the absolute value of theworking state operating current value I_(AZfs) from the formula P4 usingthe formula P3 at the particular selected operating frequency f_(S).

The relationships and formulas shown in FIG. 10 may vary slightly,depending on the approximations and assumptions made in the equivalentcircuit.

An equivalent circuit with the associated formulas is described in thedocument: Praktikum erneuerbare Energien, Versuch 3, Synchronmaschine(Practical Work in Renewable Energies, Test 3, Synchronous Machine),University of Stuttgart, ieW (Institute of Electrical EnergyConversion), as at April 2011.

A similar equivalent circuit with the corresponding formulas can befound on Wikipedia under “Synchronous motor” and the references citedthere.

Proceeding from this working state operating current value I_(AZfs), theappropriate frequency converter 40 is now ascertained in that themaximum converter current value I_(FUMAX) made available by thefrequency converter 40 must be greater than the working state operatingcurrent value I_(AZ)fs ascertained for the particular working state AZat the selected frequency f_(S).

As a further constraint for the frequency converter 40 that is to beselected, a start-up current value I_(ANLEX) for the particularrefrigerant compressor unit 20 is also used, which has likewise beenascertained experimentally and stored in a memory 58 and may, whereappropriate, be greater than the working state operating current valueI_(AZfs).

For starting up the refrigerant compressor unit 20, the frequencyconverter 40 is designed to be resistant to overload, such that amaximum converter start-up current value I_(FUANLMAX) that is greaterthan the maximum converter current value I_(FUMAX) is made availablebriefly, and for example may be 170% of the maximum converter currentvalue I_(FUMAX) for a period of three seconds.

Thus, for selection of the frequency converter 40, illustratedschematically in FIG. 9, it is relevant that the maximum convertercurrent value I_(FU) is greater than the working state operating currentvalue I_(AZ)fs, and the maximum converter start-up current valueI_(FUANLMAX) is greater than the start-up current value I_(ANLEX) of therefrigerant compressor unit 20, as illustrated for example in FIG. 3,however, the maximum converter current I_(FUMAX) and the maximumconverter start-up current I_(FUANLMAX) should be as close as possibleto the working state operating current value I_(AZfs) and the start-upcurrent value I_(ANLEX) in order to select a frequency converter withthe smallest possible maximum converter current value I_(FUMAXs), whichrepresents the most economical solution.

With a frequency converter 40 selected in this way, because of theselection method it is ensured that the frequency converter is able tooperate the refrigerant compressor unit 20 in the selected working stateAZ_(s), but a frequency converter 40 selected in this way does notensure that the refrigerant compressor 22 can consequently be operatedin all the working states AZ within the application field EF.

Rather, this approach and the selection of the frequency converter 40such that the frequency converter need only be able to supply theworking state operating current I_(AZfs) and the start-up current valueI_(ANLEX) have the effect of restricting the application field EF.

In order to display to a user the restriction of the application fieldEF that results from the selection made of the frequency converter 40,as illustrated for example in FIG. 11, and on the basis of the maximumconverter current value I_(FUMAXs) of the selected frequency converter40 s, the working states AZ in the application field EF that areassociated with this maximum converter current value I_(FUMAX) areascertained for the selected operating frequency f_(s) or indeed forother operating frequencies f_(s)′, using the equivalent circuit of thedrive motor 24′ illustrated in FIG. 10 with the known resistance valuesR and the known reactance values X from the memory 56, and using theformulas for electrical power consumption and working state operatingcurrent I_(AZ) that are illustrated in FIG. 10 and are associated withthe equivalent circuit of the drive motor 24, taking into account thepower consumption values P_(AZEX) stored in the memory 54 for thedifferent working states AZ in the application field EF at theparticular selected operating frequencies f_(s).

For this purpose, the maximum converter current value I_(FUMAXs) of theselected converter 40 s is used for the absolute value of the currentI_(AZfs) according to the formula P4, the load angle ϑ is determinedfrom this using formula P3, and the formula P2 is used to calculate thepower consumption value P_(AZCAL), and then, using the experimentalpower consumption values P_(AZEX) stored in the memory 54, all theworking states AZCAL(fs) that correspond to the calculated powerconsumption value P_(AZCAL) at the selected operating frequency fs areascertained.

The sum of these working states A_(ZCALfs) gives a boundary line G_(fs)in the application diagram 36, as illustrated in FIG. 2.

This calculation results in the boundary lines G_(fs) illustrated inFIG. 2 and FIG. 11 for different selected operating frequencies f_(s);for example the boundary line G_(fs) represents the boundary line forthe application field EF at the operating frequency f_(s) that isselected for selection of the frequency converter 40, the boundary lineG_(fr) represents for example a boundary line for the limit of theapplication field EF at a smaller operating frequency fr than theselected operating frequency fs, and the boundary line G_(frr)represents for example a boundary line of the application field EF foran operating frequency fr selected to be even smaller, and these aredisplayed by the data processing unit 50 on a display unit 53 togetherwith the application diagram 36.

Thus, a user of the method according to the invention is also at thesame time provided with information regarding the restrictions resultingfrom the selection of the frequency converter 40 in accordance with theselection method described above, and a user can check whether theserestrictions of the application field EF do or do not rule out possiblepotential working states AZ that could, where appropriate, also beapplicable for use of the refrigerant compressor unit 20.

In a fifth exemplary embodiment relating to a permanent-magnet motor, asillustrated in FIG. 12, alternatively to the first exemplary embodimentit is provided for the current I_(AZf) to be ascertained in the mannerdescribed in conjunction with the first exemplary embodiment, using thedata processing unit 50 for each experimentally determined powerconsumption value P_(AZEXf) at the particular operating frequency f,using the resistance values R and reactance values X of the equivalentcircuit that are known from FIG. 10 for each individual working stateAZ, and to be stored in a memory 54′ such that when a user makes aselection of the working state AZ_(s) and the selected operatingfrequency f_(s), the corresponding working state operating current valueI_(AZfs) may be accessed directly in the memory 56, and this workingstate operating current value I_(AZfs) corresponding to the selectedworking state AZ_(s) can be read off directly without further action,and, using the experimentally determined start-up current valueI_(ANLEX) the selection of the frequency converter 40 s can beperformed, using the maximum frequency converter currents I_(FUMAX)stored in the memory 62, in the manner already described in conjunctionwith the first exemplary embodiment.

Similarly, in the fifth exemplary embodiment, once the frequencyconverter 40 s has been established, the maximum frequency convertercurrent I_(FUMAXs) may be used to determine the working statesAZCAL_(fs) associated with this current value in the memory 54′, and todisplay the sum of all these working states AZCAL_(fs) as the particularboundary line G_(fs) for example on a display unit 64, as described inconjunction with the fourth exemplary embodiment (FIG. 13).

In a sixth exemplary embodiment, as an alternative to the fourth andfifth exemplary embodiments, it is provided, similarly to the secondexemplary embodiment, for the working state operating current valuesI_(AZf) to be determined experimentally in the memory 54′ and stored inthe memory 54′ such that in the sixth exemplary embodiment, in a similarmanner to the fifth exemplary embodiment, selection of the frequencyconverter 40 s can use the values in the memory 54′ as a starting point.

Similarly, and conversely, when determining the boundary lines G_(fs),the data processing unit 50 can proceed in accordance with the secondexemplary embodiment, with the experimentally determined working stateoperating current values I_(AZf) being stored in the memory 54′ and thenused to determine the boundary line G_(f) with the maximum convertercurrent value I_(FUMAXs) established by the selected frequency converter40 s.

Preferably, the frequency of the frequency converter 40 s used iscontrolled by a frequency control unit 70, which on the one hand detectsthe saturation temperature STE or, as an alternative, detects thesaturation pressure at the input 32 of the refrigerant compressor 22 andsupplies it to a comparator member 74, to the other side of which thereis applied a temperature specifying signal TV.

Depending on how much the saturation temperature STE deviates from thetemperature specifying signal TV, a proportional regulator 76 iscontrolled, and this generates a frequency request signal FAS that issupplied to a frequency converter controller 78 which then, in a mannercorresponding to the frequency request signal FAS, specifies thefrequency f of the frequency converter 40 s at which the drive motor 24is then operated.

If selection of the frequency converter 40 s is made in accordance withone of the exemplary embodiments described above, then, as illustratedin FIG. 3, when the refrigerant compressor unit 20 is operated, as inFIGS. 2 and 3, a working state AZ3 may occur in which the working stateoperating current I_(AZ3) as illustrated in FIG. 3 is sufficiently highfor it to happen, at frequencies f above the cut-off frequency f_(ECK),that the maximum converter current value I_(FUMAX) is already reached ata limit frequency f_(L), wherein the limit frequency f_(L) is lower thanthe operating frequency f_(s) provided for example for the working stateAZ1.

This would have the result, in a conventional structure, that thefrequency converter 40 s would switch off because of overload.

For this reason, with a frequency converter 40 according to theinvention, as illustrated in FIG. 14, a frequency-limiting unit 80 isprovided that limits the operating frequency f of the frequencyconverter 40 when it is above the cut-off frequency f_(ECK) such thatthe working state operating current value I_(AZ) does not exceed themaximum converter current value I_(FUMAXs) but at most reaches themaximum converter current value I_(FUMAXs).

This ensures that the frequency converter 40 does not switch off even inworking states that, at operating frequencies f above the cut-offfrequency f_(ECK), could result in a current of the frequency converter40 exceeding the maximum converter current value I_(FUMAX).

As illustrated in FIG. 14, the frequency-limiting unit 80 includes acurrent sensor 84 that is arranged in a supply line 72 leading from thefrequency converter 40 s to the drive motor 24 and that measures theactual working state operating current value I_(AZ) and supplies it to acomparator member 86, which compares the actual working state operatingcurrent value I_(AZ) with the maximum converter current value I_(FUMAX)as a predetermined value and supplies the comparison result to a limitregulator 92, for example a proportional regulator, which, if theworking state operating current I_(AZ) actually measured by the currentsensor 84 is greater than the maximum converter current value I_(FUMAX)serving as a reference value, generates a frequency-limiting signal fora frequency-limiting member 94 that acts on the frequency request signalFAS and prevents a further increase in the operating frequency f.

Preferably, there is additionally provided a comparator member 88coupled to the current sensor 84, which comparator member compares theworking state operating current value I_(AZ) measured by the currentsensor 84 with a maximum compressor operating current value I_(VMAX) andcontrols a limit regulator 98, for example a proportional regulator,which, when the working state operating current value I_(AZ) actuallymeasured by the current sensor 84 approximates to the maximum compressoroperating current value, I_(VMAX), likewise generates afrequency-limiting signal and transmits it to the frequency-limitingmember 94.

Preferably, the frequency-limiting signals of the limit regulators 92and 98 are compared with one another in a minimising member 102, and ineach case the frequency-limiting signal that leads to the lowest limitfrequency f_(L) is supplied to the frequency-limiting member 94.

Further, the cut-off frequency f_(ECK), which represents the minimumfrequency at which frequency restriction is performed by thefrequency-limiting member 94, is preferably also transmitted to thefrequency-limiting member 94 as a reference value.

For optimum operation of the frequency converter 40, the increase in theoutput voltage U_(FU) of the frequency converter 40 with reference tothe frequency fin the range of from f=0 to f=f_(ECK) is significant,since the increase in the output voltage U_(FU) with reference to thefrequency f of the frequency converter 40 is relevant for forming theflow in the drive motor 24.

Provided the maximum output voltage U_(FUMAX) is constant, this has theconsequence that the cut-off frequency f_(ECK) can also be constant,with the result that the increase in the output voltage U_(FU) withreference to the frequency f is likewise always constant.

If, however, with a frequency converter 40 s the supply voltagefluctuates, for example as a result of a poor-quality supply network,then the maximum output voltage U_(FUMAX) of the frequency converter 40at its output is not constant, and therefore with a constant cut-offfrequency f_(ECK) the increase in the output voltage U_(FU) wouldnecessarily vary in the frequency range between f=0 and f=f_(ECK).

In order to keep the increase in the output voltage U_(FU) withreference to the frequency constant, even with notable fluctuations inthe supply network and thus a notable fluctuation in the maximum outputvoltage U_(FUMAX) of the frequency converter 40, it is also necessary tovary the cut-off frequency f_(ECK) in a manner corresponding with thevariation in the maximum output voltage U_(FUMAX).

A conventional frequency converter 40, illustrated in FIG. 15, includesa rectifier stage 112, an inverter stage 114 and an intermediate circuit116 that is provided between the rectifier stage 112 and the inverterstage 114, across which intermediate circuit the intermediate circuitvoltage U_(Z) is applied as a DC voltage.

Here, the intermediate circuit voltage U_(Z) depends on the mainsvoltage U_(N) supplied to the rectifier stage 112, and fluctuatesproportionally to the mains voltage UN.

Here, the inverter stage 114 of the frequency converter 40 is controlledby the frequency converter controller 78, to which the frequency requestsignal FAS is supplied.

Here, on the basis of the frequency request signal FAS and with the aidof a proportional member 118, the frequency converter controller 78generates a voltage control signal SSS, which is supplied in addition tothe frequency request signal FAS to an inverter stage controller 122,which generates the output voltage U_(FU) on the basis of the frequencyrequest signal FAS and the voltage control signal SSS, which specifiesfor example percentage values of the maximum output voltage U_(FUMAX).

For adapting to drastically fluctuating mains voltages U_(N), there isthus associated with the frequency converter 40 a voltage adjustmentunit 130 that uses a voltage measuring unit 132 to measure theintermediate circuit voltage U_(Z) in the intermediate circuit 116 andsupplies this intermediate circuit voltage U_(Z) to a dividing member134, to which a reference frequency f_(REF) is also supplied.

The reference frequency f_(REF) is of a size such that, with a setpointvalue U_(ZS) of the intermediate circuit voltage U_(Z), the result isthe proportionality factor that is desired for the increase in outputvoltage U_(FU) of the converter 40 with reference to the frequency F.

The result from this dividing member 134 is supplied to a furtherdividing member 136 to which on the other hand there is supplied thedesired proportionality factor PF for the increase in output voltageU_(FU) of the frequency converter 40 with reference to the operatingfrequency f, which corresponds to the intermediate circuit voltagesetpoint value U_(ZS) divided by the reference frequency f_(REF).

The result from the second dividing member 136 is a proportionalitycorrection factor PKF which is equal to 1 if the result from the firstdividing member 134 that is supplied to this dividing member 136corresponds to the desired proportionality factor, and is not equal to 1if the intermediate circuit voltage U_(Z) differs from the intermediatecircuit voltage setpoint value U_(ZS).

If the proportionality correction factor PKF generated by the dividingmember 136 is now supplied to the proportional member 118, then it canbe used to vary the proportionality behaviour PV provided in theproportional member 118 between the operating frequency f of thefrequency request signal FAS and the voltage control signal SSS.

FIG. 16 illustrates for example how the proportionality between theoperating frequency f of the frequency request signal FAS and thevoltage control signal SSS varies.

Here, the function of the voltage adjustment unit 130 is that, when theintermediate circuit voltage U_(Z) corresponds to the intermediatecircuit voltage setpoint value U_(ZS), as illustrated in FIG. 16, thecut-off frequency corresponds to the setpoint cut-off frequencyf_(ECKSO), which is for example 50 hertz.

If the intermediate circuit voltage U_(Z) differs from the intermediatecircuit voltage setpoint value U_(ZS) by the value Δ, for example givingsmaller voltage values, then the voltage control signal SSS of 100% willbe reached at operating frequencies lower than the setpoint cut-offfrequency f_(ECKSO).

If, by contrast, the intermediate circuit voltage U_(Z) is greater thanthe intermediate circuit voltage setpoint value U_(ZS) by the value Δ,then the voltage control signal SSS of 100% will be reached at operatingfrequencies f higher than the setpoint cut-off frequency f_(ECKSO).

As illustrated in FIG. 17, this results in the cut-off frequencyf_(ECK), that is to say the frequency at which the maximum outputvoltage U_(FUMAX) is reached at the output of the frequency converter40, varying in particular in accordance with the deviation in theintermediate circuit voltage U_(Z) from the intermediate circuit voltagesetpoint value U_(ZS), with the result that the maximum output voltageU_(FUMAX) of the frequency converter 40 also varies.

1. A method for selecting a frequency converter for a refrigerant compressor unit, comprising a refrigerant compressor and an electric drive motor, a working state suitable for the operation of the refrigerant compressor unit is selected in an application field of an application diagram of the refrigerant compressor, an operating frequency is selected for this selected working state, and, on the basis of drive data, a working state operating current value corresponding to the selected working state and the selected operating frequency is ascertained for the operation of the refrigerant compressor unit.
 2. A method in accordance with claim 1, wherein, on the basis of the working state operating current value, the frequency converter of which the maximum converter current value is equal to or greater than the ascertained working state operating current value is selected from data for those frequency converters that are available for selection.
 3. A method in accordance with claim 2, wherein the frequency converter of which the maximum converter current value is closest to the working state operating current value is selected.
 4. A method in accordance with claim 1, wherein the frequency converter is selected such that its maximum converter start-up current value is equal to or greater than a start-up current value of the refrigerant compressor unit.
 5. A method in accordance with claim 4, wherein a stored start-up current value is consulted for the selection of the frequency converter.
 6. A method in accordance with claim 4, wherein the start-up current value is determined experimentally.
 7. A method in accordance with claim 4, wherein the frequency converter is selected such that its maximum converter start-up current value is as close as possible to the start-up current value.
 8. A method in accordance with claim 4, wherein the frequency converter is selected such that its maximum converter current value is as close as possible to the higher of the values of the working state operating current and start-up current.
 9. A method in accordance with claim 1, wherein the drive data are determined experimentally.
 10. A method in accordance with claim 1, wherein experimental drive data for the possible operating frequencies to be selected are stored for each working state of the refrigerant compressor unit.
 11. A method in accordance with claim 1, wherein the operating frequency to be selected lies in the range of from 0 hertz to 140 hertz.
 12. A method in accordance with claim 1, wherein the drive data comprise the experimentally determined power consumption for each working state in the application field at the various operating frequencies.
 13. A method in accordance with claim 12, wherein the working state operating current value at the selected operating frequency is calculated on the basis of the experimentally ascertained electrical power consumption at the particular operating frequency taking into consideration an equivalent circuit of the drive motor of the refrigerant compressor unit.
 14. A method in accordance with claim 1, wherein the impedance of the equivalent circuit of the drive motor is taken into consideration in order to ascertain the working state operating current value.
 15. A method in accordance with claim 14, wherein, in order to ascertain the working state operating current value, the experimentally ascertained electrical power consumption of the refrigerant compressor unit is compared with the power consumption resulting from the equivalent circuit and on this basis the slip or load angle is ascertained.
 16. A method in accordance with claim 14, wherein the working state operating current value is ascertained on the basis of the ascertained slip or load angle and the impedance of the equivalent circuit of the drive motor.
 17. A method in accordance with claim 1, wherein the working state operating current value is minimised by varying the output voltage of the frequency converter.
 18. A method in accordance with claim 1, wherein the experimentally determined electrical power consumption of each working state in the application field at the particular operating frequency is captured, in particular stored.
 19. A method in accordance with claim 1, wherein the working state operating current values calculated from the experimentally ascertained electrical power consumption are captured, in particular stored, for each working state and for each operating frequency.
 20. A method in accordance with claim 1, wherein the working state operating current value is experimentally ascertained and captured, in particular stored, for each working state and each operating frequency.
 21. A method in accordance with claim 1, wherein, on the basis of the maximum converter current value of the selected frequency converter, the working states belonging to this maximum converter current value are ascertained in the application field at a selected operating frequency with the aid of the drive data.
 22. A method in accordance with claim 21, wherein the working states ascertained in respect of the maximum converter current value are displayed visually in the application diagram.
 23. A method in accordance with claim 1, wherein there are selected only frequency converters that comprise a frequency-limiting unit, which, at operating frequencies above a cut-off frequency, limits the operating frequency in such a way that the maximum converter current value of the frequency converter is not exceeded.
 24. A method in accordance with claim 23, wherein the working state operating current value of the frequency converter is continuously detected by the frequency-limiting unit.
 25. A method in accordance with claim 23, wherein the working state operating current value of the frequency converter is compared with a current reference value, and the operating frequency is limited to a limit frequency which is present when the current reference value is reached.
 26. A method in accordance with claim 25, wherein the frequency-limiting unit takes into consideration both the maximum converter current value and also the maximum compressor operating current value as current reference value and ascertains the limit frequency on the basis of the lowest of the maximum current values.
 27. A method in accordance with claim 1, wherein there is available for selection only a frequency converter with which, by means of a voltage adjustment unit, the output voltage increases with reference to the operating frequency independently of a fluctuation of a mains voltage.
 28. A method in accordance with claim 27, wherein an intermediate circuit voltage of the frequency converter is measured and, by way of a comparison with at least one reference value, a voltage curve of the output voltage is corrected.
 29. A method carried out by a data processing unit, comprising the method steps in accordance with claim
 1. 30. A computer program product comprising commands which, when the program is run by a computer, cause the computer to carry out the method in accordance with claim
 1. 31. A data processing unit for selecting a frequency converter for a refrigerant compressor unit comprising a refrigerant compressor and an electric drive motor, the data processing unit comprises a display unit on which a working state suitable for the operation of the refrigerant compressor unit is selected in an application field of an application diagram of the refrigerant compressor, and an operating frequency is selected for this selected working state, and, on the basis of drive data stored in a memory, a working state operating current value corresponding to the selected working state and the selected operating frequency is ascertained by the data processing unit for the operation of the refrigerant compressor unit.
 32. A data processing unit in accordance with claim 31, wherein the data processing unit, on the basis of the working state operating current value, selects the frequency converter of which the maximum converter current value is equal to or greater than the ascertained working state operating current value from stored data for those frequency converters that are available for selection.
 33. A data processing unit in accordance with claim 32, wherein the data processing unit selects the frequency converter of which the maximum converter current value is closest to the working state operating current value.
 34. A data processing unit in accordance with claim 31, wherein the data processing unit selects the frequency converter is selected such that its maximum converter start-up current value is equal to or greater than a start-up current value of the refrigerant compressor unit.
 35. A data processing unit in accordance with claim 34, wherein, for the selection of the frequency converter, the data processing unit consults a start-up current value stored in a memory.
 36. A data processing unit in accordance with claim 34, wherein the start-up current value is determined experimentally.
 37. A data processing unit in accordance with claim 34, wherein the data processing unit selects the frequency converter such that its maximum converter current value is as close as possible to the start-up current value.
 38. A data processing unit in accordance with claim 34, wherein the data processing unit selects the frequency converter such that its maximum converter current value is as close as possible to the higher of the values of the working state operating current and start-up current.
 39. A data processing unit in accordance with claim 31, wherein the drive data are determined experimentally.
 40. A data processing unit in accordance with claim 31, wherein experimental drive data for the possible operating frequencies to be selected are stored in the memory for each working state of the refrigerant compressor unit.
 41. A data processing unit in accordance with claim 31, wherein the operating frequency to be selected lies in the range of from 0 hertz to 140 hertz.
 42. A data processing unit in accordance with claim 31, wherein the drive data comprise the experimentally determined power consumption for each working state in the application field at the various operating frequencies.
 43. A data processing unit in accordance with claim 42, wherein the data processing unit calculates the working state operating current value at the selected operating frequency on the basis of the experimentally ascertained electrical power consumption at the particular operating frequency taking into consideration an equivalent circuit of the drive motor of the refrigerant compressor unit.
 44. A data processing unit in accordance with claim 31, wherein the impedance of the equivalent circuit of the drive motor is taken into consideration by the data processing unit in order to ascertain the working state operating current value.
 45. A data processing unit in accordance with claim 42, wherein, in order to ascertain the working state operating current value, the data processing unit compares the experimentally ascertained electrical power consumption of the refrigerant compressor unit with the power consumption resulting from the equivalent circuit and on this basis ascertains the slip or load angle.
 46. A data processing unit in accordance with claim 44, wherein the data processing unit ascertains the working state operating current value on the basis of the ascertained slip or load angle and the impedance of the equivalent circuit of the drive motor.
 47. A data processing unit in accordance with claim 31, wherein this varies the output voltage of the frequency converter in order to ascertain the working state operating current value, and, for the selection of the frequency converter, takes into consideration the output voltage of the frequency converter corresponding to a minimum of the working state operating current value.
 48. A data processing unit in accordance with claim 31, wherein the data processing unit stores the experimentally determined electrical power consumption of each working state in the application field at the particular operating frequency.
 49. A data processing unit in accordance with claim 31, wherein the data processing unit stores the working state operating current values calculated from the experimentally ascertained electrical power consumption for each working state and for each operating frequency.
 50. A data processing unit in accordance with claim 31, wherein the working state operating current value is experimentally ascertained and stored by the data processing unit for each working state and for each operating frequency.
 51. A data processing unit in accordance with claim 31, wherein the data processing unit, on the basis of the maximum converter current value of the selected frequency converter, ascertains the working states belonging to this maximum converter current value in the application field at a selected operating frequency with the aid of the drive data.
 52. A data processing unit in accordance with claim 51, wherein the data processing unit displays the working states ascertained in respect of the maximum converter current value on the display unit in the application diagram.
 53. A data processing unit in accordance with claim 31, wherein the data processing unit selects only frequency converters that comprise a frequency-limiting unit, which, at operating frequencies above a cut-off frequency, limits the operating frequency in such a way that the maximum converter current value of the frequency converter is not exceeded.
 54. A data processing unit in accordance with claim 53, wherein the working state operating current value of the frequency converter is continuously detected by the frequency-limiting unit.
 55. A data processing unit in accordance with claim 53, wherein the working state operating current value of the frequency converter is compared with a current reference value, and the operating frequency is limited to a limit frequency which is present when the current reference value is reached.
 56. A data processing unit in accordance with claim 55, wherein the frequency-limiting unit considers both the maximum converter current value and also the maximum compressor operating current value as current reference value and ascertains the limit frequency on the basis of the lowest of the maximum current values.
 57. A data processing unit in accordance with claim 31, wherein there is available for selection by the data processing unit only a frequency converter with which, by means of a voltage adjustment unit, the output voltage increases with reference to the operating frequency independently of a fluctuation of a mains voltage.
 58. A data processing unit in accordance with claim 57, wherein an intermediate circuit voltage of the frequency converter is measured and, by way of a comparison with at least one reference value, a voltage curve of the output voltage is corrected.
 59. A refrigerant compressor system comprising a refrigerant compressor unit having a refrigerant compressor and an electric drive motor and also comprising a frequency converter for operating the electric drive motor, the frequency converter comprises a frequency-limiting unit, which at operating frequencies above a cut-off frequency limits the operating frequency in such a way that the maximum converter current value of the frequency converter is not exceeded.
 60. A refrigerant compressor system in accordance with claim 59, wherein the working state operating current value of the frequency converter is detected continuously by the frequency-limiting unit.
 61. A refrigerant compressor system in accordance with claim 59, wherein the working state operating current value of the frequency converter is compared with a current reference value, and the operating frequency is limited to a limit frequency which is present when the current reference value is reached.
 62. A refrigerant compressor system in accordance with claim 61, wherein the frequency-limiting unit takes into consideration both the maximum converter current value and also the maximum compressor operating current value as current reference value and determines the limit frequency on the basis of the lowest of the maximum current values.
 63. A refrigerant compressor system comprising a refrigerant compressor unit having a refrigerant compressor and an electric drive motor and also comprising a frequency converter for operating the electric drive motor, the frequency converter comprises a voltage adjustment unit which controls an increase of the output voltage with reference to the operating frequency such that this increase occurs independently of a fluctuation of a mains voltage.
 64. A refrigerant compressor system in accordance with claim 63, wherein the voltage adjustment unit measures an intermediate circuit voltage of the frequency converter and, by way of a comparison with at least one reference value, corrects the increase of the output voltage.
 65. A refrigerant compressor system in accordance with claim 64, wherein the voltage adjustment unit generates a proportionality correction factor with which the increase in the output voltage of the frequency converter is corrected.
 66. A refrigerant compressor system in accordance with claim 64, wherein the reference values used by the voltage adjustment unit comprise at least one of the values as: a reference frequency, a proportionality factor, and an intermediate circuit voltage setpoint value.
 67. A refrigerant compressor system in accordance with claim 63, wherein the frequency converter comprises a frequency converter controller, which on the basis of a frequency request signal generates a voltage control signal which is fed, in addition to the frequency request signal, to an inverter stage controller of an inverter stage of the frequency converter, and in that the voltage adjustment unit cooperates with the frequency converter controller in order to control the increase in the output voltage with reference to the operating frequency.
 68. A refrigerant compressor system in accordance with claim 67, wherein the frequency converter controller has a proportional member which, on the basis of the frequency request signal, generates the voltage control signal, and in that the voltage adjustment unit corrects a proportionality behaviour of the proportional member.
 69. A refrigerant compressor system in accordance with claim 68, wherein the proportionality behaviour of the proportional member is corrected using the proportionality correction factor. 